LPC1769 and voltage divider for USB VBUS

[lpcware]

Content originally posted in LPCWare by robert hulsebos on Fri Dec 11 09:04:49 MST 2015
Hi all,

We have a PCB that uses the LCP1769 and configures its USB port as an USB device. It is a self-powered design, the LPC1769 does not derive its power from the USB bus.
In more recent versions of the LPC176x datasheet / AN11392 it is recommended that two resistors are added to the VBUS signal of the USB port. These resistors are to function as a voltage-divider for bringing down the 5V of the VBUS signal to the maximum allowed 3.6V. (see paragraph 5.1 of most recent datasheet)
As our PCB design predates these recommendations, it does not include the voltage-divider. However, it does include a series resistor of 1K in the line to the VBUS pin.
My questions now are:

1. Is VBUS voltage-divider really required ? What is the real risk of not having the voltage-divider ?
2. Is the 1K series resistor a sufficient means of mitigating this risk ?
3. What are the recommended values for the resistors of the voltage-divider ?

I noticed in other forums on this site that for other LPC families NXP recommendations are [u]not[/u] to use a voltage-divider, but other circuity instead.
( e.g. see https://www.lpcware.com/content/forum/usb0-vbus-voltage-divider )

3. Does this now also apply for the LPC1769 - has NXP insight on this matter changed?

Regards,

Robert

[lpcware]

Content originally posted in LPCWare by robert hulsebos on Thu Dec 17 05:37:03 MST 2015
Thanks for your response!
I understand reason behind the voltage divider. The thing is we use the USB function in self-powered design and only recently found out that the voltage divider is recommended. At the time of our original design there was no voltage divider in the NXP documentation... I asked the questions because I am unsure as to we should do a redesign of the PCB for including the voltage divider, or if we could continue to use the existing PCB design without the voltage divider. With acceptable risk. I hope somebody from NXP can elaborate on the risks involved so I can make a decision...

[lpcware]

Content originally posted in LPCWare by masaz on Mon Dec 14 05:32:15 MST 2015
NXP USB application note suggests 24k to VBUS and 39k to ground, input from between. I have used 4.7k and 10k.

See:
http://www.nxp.com/documents/application_note/AN11392.pdf

[lpcware]

Content originally posted in LPCWare by masaz on Mon Dec 14 05:23:16 MST 2015
Hi,

I have had the understanding that while most I/O pins are 5V tolerant, this applies
only when the device is powered up, while in USB self powered design, the
device can be connected to USB without power. Thus, it is suggested to use the
voltage divider.

In addition, the same pin can be used to analog function (ADC) which is never
5 V tolerant.

Br, Matti